Answers

I don't want to mislead you by attempting to interpret the certification guidelines. I suggest you read the certification requirements as a whole rather than trying to cherry pick items out of context.

That said, the statement you quote doesn't appear to say anything allowing inter process communication. From a technical sense, local network communication is deliberately blocked for Windows Store apps.

"Unless your app is a Storefront for software, your app must not encourage users to install desktop software
that is not required to complete the primary value of the app."

Does it mean that if installing a desktop app is necessary to complete the primary value of the app then we could ask user to install a desktop app as well? Is a network(TCP/IP) communication to the desktop app acceptable in that case?

I don't want to mislead you by attempting to interpret the certification guidelines. I suggest you read the certification requirements as a whole rather than trying to cherry pick items out of context.

That said, the statement you quote doesn't appear to say anything allowing inter process communication. From a technical sense, local network communication is deliberately blocked for Windows Store apps.